The invention generally relates to metal air electrochemical cells.
Batteries are commonly used electrical energy sources. A battery contains a negative electrode, typically called the anode, and a positive electrode, typically called the cathode. The anode contains an active material that can be oxidized; the cathode contains or consumes an active material that can be reduced. The anode active material is capable of reducing the cathode active material. In order to prevent direct reaction of the anode material and the cathode material, the anode and the cathode are electrically isolated from each other by a sheet-like layer, typically called the separator.
When a battery is used as an electrical energy source in a device, such as a hearing aid or a cellular telephone, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge.
One configuration of a battery is a button cell, which has the approximate size and cylindrical shape of a button. In a button cell, the container for the anode and the cathode includes a lower cup-like structure, called the cathode can, and an upper cup-like structure retained within the cathode can, called the anode can. The anode can and the cathode can are separated by an insulator, such as an insulating gasket or seal. The anode can and the cathode can are crimped together to form the container.
In a metal air electrochemical cell, oxygen is reduced at the cathode, and a metal is oxidized at the anode. Oxygen is supplied to the cathode from the atmospheric air external to the cell through air access ports in the container. Oxygen diffuses through the cathode structure to the reaction zone. Oxygen reduction requires a three phase reaction zone consisting of air, the electrolyte, and a carbon-catalyst component. The cathode must be wetted in order for oxygen reduction to occur. If too much wetting occurs, however, the cathode can wet-through entirely. When the cathode wets through, the cell polarizes, i.e., the power output from the cell drops precipitously; in addition, the electrolyte solution can leak through to the exterior of the cell.
In general, the invention relates to cathodes for metal air electrochemical cells, and methods for making these cathodes. Electrochemical cells made with cathodes of the invention are relatively resistant to wet-through problems and, at the same time, are capable of generating high currents. The methods and compositions of the present invention can therefore be used to make metal air cells having good performance characteristics, especially in high rate applications.
In one aspect, the invention features a cathode for a metal air electrochemical cell; the cathode includes (a) a first layer including about 30% to about 70% of an organic polymer, such as polytetrafluoroethylene, by weight; (b) a second layer including about 10% to about 30% of an organic polymer, such as polytetrafluoroethylene, by weight; and (c) a catalyst. The first layer and the second layer contact each other at a textured interface.
A xe2x80x9ctexturedxe2x80x9d interface includes at least one textured surface. A textured surface is one that is not smooth, i.e., one that is roughened, and therefore has a higher surface area than a smooth surface of the same dimensions. It is thus the macroscopic texture that is referenced here, as opposed to the microscopic texture observed on individual particles in the electrode. The geometric surface area of a textured surface is preferably at least 10% higher, and more preferably at least 25% or 50% higher, than that of a smooth, flat surface of the same dimensions.
The dual layer textured cathode of the invention offers several advantages. The layer adjacent to the anode gel has a relatively low concentration of an organic polymer such as polytetrafluoroethylene (PTFE). The low concentration of PTFE makes this layer relatively hydrophilic, and the hydrophilicity of this layer promotes wetting. Sufficient wetting of this layer is important for oxygen reduction. The layer adjacent to the air access ports has a higher concentration of PTFE, making it hydrophobic. The hydrophobicity of this layer helps to prevent the cathode from wetting through. The two layers contact each other at an interface, which is textured to provide a higher surface area for the three phase (i.e., air, electrolyte, and catalyst) reaction zone. The higher surface area improves cell performance during high rate discharge.
The cathode can be used in the preparation of an electrochemical cell having very good discharge capacities.
In another aspect, the invention features a method of making a cathode for an electrochemical cell that includes combining carbon with AgMnO4 to form a mixture, then preparing a cathode with the mixture. The cathode can be used to prepare a metal air cell or an alkaline air cell.
The use of AgMnO4 as a catalyst precursor offers numerous advantages. For example, the decomposition of AgMnO4 results in fine dispersions of MnO2 and Ag. Ag facilitates the direct reduction of O2 to 4 OHxe2x88x92. This is useful because manganese dioxide cannot participate in the 4 exe2x88x92 reduction of O2 to 4 OHxe2x88x92. Those oxygen atoms that are not reduced to OHxe2x88x92 in the 4 exe2x88x92 process may be reduced to peroxide (HO2xe2x88x92) in a two e31  process. Rapid elimination of this peroxide will enable a higher running voltage for the cathode. Both Ag and MnO2 are effective peroxide elimination catalysts. A further advantage is that silver improves the conductivity of the cathode.
In another aspect, the invention features a cathode for a metal air electrochemical cell; the cathode contains manganese and silver and is substantially free of potassium. By xe2x80x9csubstantially free of potassium,xe2x80x9d it is meant that the cathode is substantially free of potassium before the cathode contacts the electrolyte in the cell. The cathode preferably contains less than about 7 percent by weight silver or less than about 3 percent by weight silver.
In another aspect, the invention features a cathode for a zinc air cell that has a current density of at least 70 MA/cm2 at a voltage of xe2x88x920.25 volts, versus a Hg/HgO reference. Preferred cathodes can have current densities of at least 80, 90, 100, or 150 mA/cm2 at a voltage of xe2x88x920.25 volts, versus a Hg/HgO reference.
In another aspect, the invention features a cathode for an electrochemical cell. The cathode includes a catalyst, and is prepared using AgMnO4 as a catalyst precursor.
Other features and advantages of the invention will be apparent from the description of the preferred embodiment thereof, and from the claims.